The present invention relates to an engine exhaust purification device provided with a catalyst.
JP-A-H9-228873 published by the Japanese Patent Office in 1997 discloses a technique wherein an oxygen amount stored in a three-way catalyst (hereafter, xe2x80x9coxygen storage amountxe2x80x9d) is estimated based on an engine intake air amount and an air fuel ratio of an exhaust flowing into the catalyst, and engine air-fuel ratio control is performed so that the oxygen storage amount of the catalyst is constant.
To maintain the NOx (nitrogen oxides), CO and HC (hydrocarbon) conversion efficiency of the three-way catalyst at a maximum, the catalyst atmosphere must be maintained at the stoichiometric air-fuel ratio. If the oxygen storage amount of the catalyst is maintained constant, oxygen in the exhaust is stored in the catalyst even if the air-fuel ratio of the exhaust flowing into the catalyst temporarily becomes lean, and conversely, oxygen stored in the catalyst is released even if the air-fuel ratio of the exhaust flowing into the catalyst temporarily becomes rich, so the catalyst atmosphere can be maintained at the stoichiometric air-fuel ratio.
Therefore, in an exhaust purification device performing this type of control, it is required to calculate the oxygen storage amount precisely to maintain the conversion efficiency of the catalyst at a high level, and various methods of computing the oxygen storage amount have been proposed.
However, even if the oxygen storage amount can be precisely computed, if the catalyst temperature on engine restart is high such as when the elapsed time from when the engine stopped on the immediately preceding occasion is short, NOx tends to be easily released.
This is due to the fact that, if the catalyst temperature on engine startup is high, the catalyst has already stored a large amount of oxygen which has entered from the exhaust passage outlet and diffused in the exhaust passage, so the NOx purification performance of the catalyst decreases. When the oxygen storage amount of the catalyst is large, and the air-fuel ratio of the inflowing exhaust is shifted to lean, the catalyst atmosphere cannot be corrected to the stoichiometric air-fuel ratio, and NOx in the exhaust cannot be completely purified.
It is therefore an object of this invention to suppress the NOx release amount on engine startup in an exhaust purification device which controls the air-fuel ratio of an engine to maintain the oxygen storage amount of a catalyst at a fixed level.
In order to achieve above object, this invention provides an exhaust purification device for an engine, comprising a first catalyst provided in an exhaust passage of the engine, a front sensor which detects the characteristics of the exhaust flowing into the first catalyst, and a microprocessor programmed to determine whether the engine starts up from a warmed-up state when the engine starts, control the air-fuel ratio of the engine to rich until the exhaust flowing out from the first catalyst has become rich when it is determined that the engine starts up from the warnmed-up state, compute the oxygen storage amount of the first catalyst based on the characteristics of the exhaust flowing into the first catalyst, and control the air-fuel ratio of the engine based on the computed oxygen storage amount so that the oxygen storage amount of the first catalyst is a target value.
According to an aspect of the invention, this invention provides an exhaust purification device for an engine, comprising a first catalyst provided in an exhaust passage of the engine, a second catalyst provided downstream of the first catalyst, a front sensor which detects the characteristics of the exhaust flowing into the first catalyst, and a microprocessor programmed to determine whether the engine starts up from a warmed-up state when the engine starts, control the air-fuel ratio of the engine to rich until the exhaust flowing out from the second catalyst has become rich when it is determined that the engine starts from the warmed-up state, compute the oxygen storage amount of the first catalyst based on the characteristics of the exhaust flowing into the first catalyst, and control the air-fuel ratio of the engine based on the computed oxygen storage amount so that the oxygen storage amount of the first catalyst is a target value.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.
Strictly speaking, noble metals adsorb oxygen in the molecular state, and oxygen storage materials absorb oxygen as compounds, but in the following description, adsorption and absorption will be collectively referred to as storage.
Further, the expression xe2x80x9cthe exhaust air-fuel ratio is richxe2x80x9d means that the oxygen concentration in the exhaust is lower than the oxygen concentration in the exhaust when the engine is running at the stoichiometric air-fuel ratio, and the expression xe2x80x9cthe exhaust air-fuel ratio is leanxe2x80x9d means that the oxygen concentration in the exhaust is higher than the oxygen concentration in the exhaust when the engine is running at the stoichiometric air-fuel ratio. The expression xe2x80x9cthe exhaust air-fuel ratio is stoichiometricxe2x80x9d means that the oxygen concentration of the exhaust is equal to the oxygen concentration in the exhaust when the engine is running at the stoichiometric air-fuel ratio.